4.3 OMC Does not start, lots of backfires though...

[offroadwonder]

After over a month of tinkering and frustrating my wife, I have finally gotten my project boat ('96 Chris Craft Concept 18 w/ 4.3GL) ready to try to start. I bought the boat with a frozen motor that I tore down intending to rebuild but ended up visiting the local scrap yard for a donor motor instead. I got the new motor (4.3L from a '95 Chevy Blazer) installed and everything reassembled. I finished up the wiring work today so that I can crank the motor.

Put gas in it, primed the new fuel filter and sprinkled a little fuel in the carb for good measure, crossed my fingers and cranked away. It just backfires like mad both through the exhaust and through the carb. My first thought is timing, but I don't know how to get it any closer for initial startup then just aligning the rotor to #1 spark plug at TDC.

Any advice would be much appreciated. At this point, I am pretty sure all my neighbors think I'm conducting explosive experiments in my driveway with all the backfires coming from my house.

JB

 

[Silvertip]

Re: 4.3 OMC Does not start, lots of backfires though...

It is either a timing or firing order problem -- assuming of course the fuel system is delivering fuel as it should.

 

[PINESOL]

Re: 4.3 OMC Does not start, lots of backfires though...

try turning the distributor a little bit ... be sure the wires are in proper order . and let us know ...

 

[wncrjb]

Re: 4.3 OMC Does not start, lots of backfires though...

When you brought #1 up on TDC, was it on compression stroke? or 180 degrees off.

 

[offroadwonder]

Re: 4.3 OMC Does not start, lots of backfires though...

It is definitely getting fuel, I was actually afraid it was getting too much fuel when you go WOT the accellerator pump dumps enough fuel in that it puddles in the intake (by the way, i rebuilt the carb while it was off...so I don't think anything is wrong in there).

After a little further research I get the feeling I must have gotten the timing 180 deg off. I set TDC based on the timing marks on the crank and cam when I had the cover off, but I never verified that the valves were actually closed. Tomorrow (too bad its too late tonight) I will check TDC by taking out #1 spark plug and retime.

Does that sound about right?

JB

 

[Coors]

Re: 4.3 OMC Does not start, lots of backfires though...

Was a new timing chain involved? Is the dist 180 out?

 

[offroadwonder]

Re: 4.3 OMC Does not start, lots of backfires though...

She lives! I pulled the #1 spark plug and found TDC for sure and sure enough, the dist. rotor was facing 180 deg the wrong way. Fixed that and double checked I had the firing order right and she fired right up! Apparently just because the timing marks on the crank and cam gears are lined up doesn't mean the motor is at TDC (lesson learned).

Now I just have to figure out how to make a timing shunt so I can set the timing correctly. Thanks for all the responses.

JB